The present invention relates to a silver halide photographic emulsion and a process for preparing the photographic emulsion. More particularly, the present invention relates to a silver halide photographic emulsion comprising triangular tabular grains having an extremely small thickness and a high aspect ratio.
Tabular silver halide grains (hereinafter referred to as xe2x80x9ctabular grainsxe2x80x9d) have the following photographic properties.
(1) The ratio of surface area to volume (hereinafter specific surface area) of tabular silver halide grains is great. Further, tabular silver halide grains can adsorb a large amount of a sensitizing dye on the surface thereof. Therefore, tabular silver halide grains exhibit a relatively high spectral sensitization sensitivity than intrinsic sensitivity.
(2) When an emulsion comprising tabular grains is applied to a support and dried, these tabular grains are oriented parallel to the surface of the support. Therefore, a thin coating layer can be formed. The resulting photographic light-sensitive material exhibits a good sharpness.
(3) In X-ray photographic system, when a sensitizing dye is added to tabular grains, silver halide crossover light can be drastically reduced, making it possible to inhibit the deterioration of image quality.
(4) Tabular grains scatter little light, making it possible to obtain an image having a high resolving power.
(5) Tabular grains have a low sensitivity to blue light. Therefore, when tabular grains are incorporated in a green-sensitive layer or red-sensitive layer, an emulsion free of yellow filter can be used.
Thus, tabular grains have many advantages and hence have heretofore been incorporated in commercially available high sensitivity photographic light-sensitive materials. JP-B-6-44132 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) and JP-B-5-16015 disclose an emulsion of tabular grains having an aspect ratio of 8 or more. The term xe2x80x9caspect ratioxe2x80x9d as used herein means the ratio of diameter to thickness of tabular grain. The term xe2x80x9cdiameter of grainxe2x80x9d as used herein means the diameter of the circle having the same area as the projected area of grain determined under microscope or electron microscope. The term xe2x80x9cthickness of grainxe2x80x9d as used herein means the distance between two parallel main planes constituting tabular silver halide grain.
JP-B-4-36374 discloses a color photographic light-sensitive material having improvements in sharpness, sensitivity and graininess obtained by incorporating tabular grains having a thickness of less than 0.3 xcexcm and a diameter of 0.6 xcexcm or more in at least one of green-sensitive emulsion layer and red-sensitive emulsion layer.
In recent years, however, silver halide photographic materials have had higher sensitivity and smaller format. It has thus keenly been desired to provide color photographic light-sensitive materials having higher sensitivity and better image quality. Therefore, an emulsion of silver halide grains having higher sensitivity and better graininess has been required. However, the conventional emulsion of tabular silver halide grains leave something to be desired in meeting these requirements. Further improvements in properties have been required.
As the aspect ratio of tabular grains is increased, the specific surface area of the grains is increased, making the best use of the advantages of tabular grains. In other words, a greater amount of a sensitizing dye can be adsorbed by a greater surface area to increase the absorption of light per grain, making it possible to obtain a high sensitivity. Thus, studies have heretofore been made of preparation of thinner tabular grains. JP-B-5-12696 discloses a process for preparing thin tabular grains involving the use of a gelatin rendered ineffective by oxidizing methionine group contained therein as a dispersant. JP-A-8-82883 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a process for preparing thin tabular grains involving the use of a gelatin having amino group and methionine group rendered ineffective as a dispersant. JP-A-10-148897 discloses a process for preparing thin tabular grains involving the use of a gelatin having amino group chemically modified and at least two carboxyl groups incorporated therein as a dispersant.
It has been well-known that tabular grains having (111) face as a main plane include triangular tabular grain, hexagonal tabular grain and tabular grain having a shape intermediate therebetween. The provision of tabular grains having a uniform shape has been the target of past studies in quest of excellent photographic properties such as higher sensitivity and higher gradation. For the details of hexagonal tabular grains having (111) face as a main plane, reference can be made to JP-B-5-61205. Referring to triangular tabular grains having (111) face as a main plane, on the other hand, JP-A-3-238443 discloses that an emulsion comprising tabular grains in a proportion of 50% or more in all grains as calculated in terms of projected area contains monodisperse triangular tabular grains.
It is merely described in examples that the proportion of triangular tabular grains in all grains contained in the emulsion as calculated in terms of projected area falls less than 30%. EP 754964A1 discloses an emulsion comprising tabular silver halide grains having a thickness of less than 0.3 xcexcm and an equivalent circle diameter of 0.6 xcexcm or more as calculated in terms of circle in a proportion of at least 30% of all grains as calculated in terms of projected area. The above described publication discloses an example involving the preparation of triangular tabular grains using silica sol as a dispersant in the presence of a phosphonium salt. However, it is described that the triangular tabular grains account for 53 at maximum by number or 66 at maximum by area and exhibit an average thickness of 0.08 xcexcm at minimum and a maximum aspect ratio of 18. As a result, the proportion of triangular tabular grains is still low, and the thickness of tabular grains is not sufficient yet.
It is therefore an object of the present invention to provide a silver halide emulsion comprising triangular tabular grains having an extremely small thickness, an extremely high aspect ratio and (111) face as a main plane.
The foregoing object of the present invention will become apparent from the following detailed description and examples.
The foregoing object of the present invention is accomplished by the following aspects of the present invention.
(1) A silver halide photographic emulsion comprising triangular tabular grains having (111) face as a main plane and a silver bromide content of 70 mol-% or more incorporated therein in a proportion of 60% or more of all silver halide grains contained therein as calculated in terms of projected area, the triangular tabular grains each having a thickness of less than 0.05 xcexcm and an equivalent circle diameter of 0.6 xcexcm or more as calculated in terms of circle.
(2) A process for preparing a silver halide photographic emulsion according to the above item (1) which comprises steps of nucleation, ripening and growth, wherein at least one of compounds represented by formulae (I), (II) and (III) is not present during nucleation but is present during ripening and growth: 
wherein R1 represents an alkyl group, alkenyl group or aralkyl group; R2, R3, R4, R5 and R6 each represent a hydrogen atom or substituent; R2 and R3, R3 and R4, R4 and R5 and R5 and R6 may be condensed to each other, with the proviso that at least one of R2, R3, R4, R5 and R6 represents an aryl group; and Xxe2x88x92 represents a counter anion; 
wherein A1, A2, A3 and A4 may be the same or different and each represent a nonmetallic element required to form a nitrogen-containing heterocyclic group; B represents a divalent connecting group; m represents an integer of 0 or 1; R1, and R2 each represent an alkyl group; X represents an anion; and n represents an integer of from 0, or 2, with the proviso that when the compound is an intramolecular salt, n is 0 or 1.
(3) The process for preparing a silver halide photographic emulsion according to the above item (2), which comprises supplying an aqueous solution of a water-soluble silver salt and an aqueous solution of a water-soluble halide into a mixer provided outside a reaction vessel for causing nucleation and/or growth of silver halide grains where they are then mixed to form finely divided silver halide grains, and then immediately supplying the finely divided silver halide grains into the reaction vessel where the nucleation and/or growth of silver halide grains is effected.
(4) The process for preparing a silver halide photographic emulsion according to the above item (2) or (3), wherein at least one of compounds represented by formulae (I), (II) and (III) is continuously supplied into the reaction vessel during the growth of grains.